1. Field of the Invention
The present invention relates to a method and an apparatus for estimating a length of a cable, and more particularly to, a method and an apparatus for estimating a length of a cable in Ethernet.
2. Description of the Prior Art
Ethernet is standardized as IEEE 802.3. So far, it has been widely used in local area networks (LANs). In order to meet all kinds of requirements, Ethernet devices are connected by different network media such as twisted pair cables, coaxial cables, optical fibers and etc., to support the bit rates of 10 Mbps, 100 Mbps, and 1 Gbps.
As known so far, the Ethernet devices are connected by the twist pair cables, and support the bit rates of 10 Mbps and 100 Mbps. At the beginning of connection, two mutually connected Ethernet devices confirm their connection ability and data rate through a link pulse. For example, the connection can run at 100 Mbps only if both Ethernet devices support the bit rate of 100 Mbps. Otherwise, the lower bit rate, 10 Mbps, is adopted.
On the other hand, in order to reduce the power consumption in the network system, the prior art usually takes advantage of time domain reflectometry (TDR) to detect the length of the cable as a power reference of data transmission. Since impedance variance at the ends of the cable causes reflection of the input pulse, TDR can thus calculate the length of the cable or determine defect positions of the cable based on travel time and velocity of the reflection signal.
However, as known by those skilled in the art, if the cable is too short (i.e. the end of the cable is too close to the transmission terminal), the travel time of the reflection signal decreases dramatically, the reflection signal therefore overlaps with the incident signal. Thus, the reflection signal may not be distinguishable, resulting in blind spots on cable length detection. In this situation, the prior art usually narrows the pulse width of the incident signal for enhancing the resolution of the reflection signal.
Since narrowing the pulse width of the incident signal accompanies power reduction of the incident signal, it is possible that the prior art cannot detect the refection signal due to power attenuation when the end of the cable is far from the transmission terminal. In addition, the narrowed pulse signal no longer meets the Ethernet standards, and thus requires extra hardware to carry out.